This invention relates to an internal combustion engine and more particularly to a variable valve timing mechanism for operating the valves of such an engine.
It is well known that one of the factors that controls the performance of a four cycle, internal combustion engine is the valve timing mechanism. Generally, the valves are operated by one or more camshafts at a timed relationship to the rotation of the engine output shaft. Frequently, the intake valves are operated by a different camshaft than the camshaft which operates the exhaust valves. Such engines are called "double overhead cam engines",(DOHC).
Generally, the optimal valve timing for an engine varies, depending upon the speed and load at which the engine is operating. Thus, conventional engines having fixed valve timing arrangement generally are designed to provide a comprise between good running at low speeds and low loads and maximum engine output. Depending upon the use of the engine, the bias may be toward one or the other end of these two alternative ranges.
In order to improve performance over a wider range of engine speed and load conditions, it has been proposed to employ a variable valve timing arrangement in the drive for the camshafts. In this way, the timing relationship of the camshafts can be adjusted so as to provide optimal performance for more running condition.
The variable valve timing mechanisms which have been proposed generally fall into two categories. With the first of these and the simpler arrangement, the timing of both camshafts is generally altered in the same direction and at the same degree. This is done by interposing one variable valve timing mechanism in the timing drive between the engine output shaft and the camshafts. This has the advantages of simplicity, lower cost and still provides greater flexibility in engine performance.
The other type of system includes a pair of variable valve timing mechanisms each of which is interposed between the drive for the respective of the camshafts from the engine output shaft. This obviously doubles the number of components, including the control mechanism. It does, however, offer the possibility of a greater flexibility in overall engine performance.
It has been discovered, however, that there are a number of running conditions where the performance is optimal if both camshafts are adjusted at the same phase angle. Other running conditions have been found to require a different adjustment in the timing between the camshafts. Although this can be employed easily in an arrangement wherein there are independent variable valve timing mechanisms associated with each camshaft, this makes the control strategy more complicated.
It is, therefore, a principal object of this invention to provide an improved variable valve timing mechanism for an engine wherein two camshafts may have their timing altered simultaneously or independently of each other, depending upon the running characteristics.
It is a further object of this invention to provide an improved camshaft variable valve timing mechanism wherein simultaneous adjustment of both camshafts can be accomplished through the use of one variable valve timing mechanism and adjustment of the timing of the camshafts relative to each other is done by a separate variable valve timing mechanism.